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| Titel |
Relationship between atmospheric methane lifetime, isotope budget and effective sink enrichments simulated in AC-GCM EMAC |
| VerfasserIn |
Sergey Gromov, Benedikt Steil |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250134400
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| Publikation (Nr.) |
 EGU/EGU2016-15123.pdf |
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| Zusammenfassung |
| In his note adamant for interpreting paleoclimate isotope-resolved CH4 records, Tans [1] has
emphasised the large disparity in the timescales of abundance and isotope ratio changes in the
atmospheric CH4. Derived using a simple two-box model, quantitatively this result is
consistent for hemispherically average (homogeneous) CH4 emitted and removed by yet
homogenous and invariable sources and sinks. However, neither the abundance of
methane nor its sources and sink rate (determined largely by OH and temperature) are
spatiotemporally even. The situation is further complicated by non-linear convolution of
photochemistry and mixing/transport acting between source regions and a regarded
location. Compared to about 10 years on average in the troposphere, local CH4 lifetime
varies from 15 months (near the surface in tropics) to hundreds of years at high
latitudes in winter. How does the local isotope enrichment of CH4 (resulting from sink
fractionation processes) correspond to that? Will using a realistic atmospheric model
indicate importance of the abovementioned issues, and for which paleoclimate
records?
Inspired by these questions, we designed a similar to [1] experiment implemented,
however, in the 3D AC-GCM model EMAC [2; 3] which resolves 13C/12C and 2H/1H isotope
chemistry, 14CH4 abundance and methane photochemical sinks including reactions with
OH, O(1D), Cl with respective kinetic isotope effects up to the middle atmosphere
(about 80 km). We simulate long-term equilibration of CH4 abundance and isotope
ratios for several emission magnitudes/distributions and OH fields, subsequently
perturbed by the pulse change in source strengths or isotope signatures. The resulting
sensitivities of effective 13C/12C and 2H/1H enrichments in atmospheric methane
(13Cɛ and 2Hɛ, respectively) are important for gauging the isotope signatures of
CH4 sources derived for present and from paleo-records of CH4. The simulated
hemispheric difference in 13Cɛ correspond to that of [1] when averages are used, however
differences in local values (e.g. between the N and S poles) may reach double of that. We
find that surface ɛ values can be parametrically derived using local and average
tropospheric CH4 mixing ratios, however not lifetimes. Importantly, the effective
enrichment signal is lost if the lower boundary condition (so-called “nudging”)
is used instead of surface CH4 emissions in the model. Such will likely lead to
wrong estimates of the isotope signatures of CH4 sources in inverse modelling
approaches. Some conclusions and quantitative estimates of 2Hɛ are presented in
addition.
1. Tans, P. P.: A note on isotopic ratios and the global atmospheric methane budget, Glob.
Biogeochem. Cyc., 11, 77-81, doi: 10.1029/96gb03940, 1997.
2. Jöckel, P., et al.: Development cycle 2 of the Modular Earth Submodel System
(MESSy2), Geosci. Model Dev., 3, 717-752, doi: 10.5194/gmd-3-717-2010, 2010.
3. Gromov, S., et al.: A kinetic chemistry tagging technique and its application to
modelling the stable isotopic composition of atmospheric trace gases, Geosci. Model Dev., 3,
337-364, doi: 10.5194/gmd-3-337-2010, 2010. |
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